Welcome to astronomy, one of the oldest sciences and still one of the most exciting. In the first episode of Crash Course Astronomy, above, Phil Plait (the “Bad Astronomer”) answers a simple question: what is astronomy? This article covers the same ground, from who studies the sky to how the field began, and ends with ways you can start exploring it yourself.
A simple definition
Astronomy is the scientific study of everything outside Earth’s atmosphere: the Moon, the Sun, planets, asteroids and comets, stars, nebulae, galaxies and the universe as a whole. Astronomers want to know what these objects are made of, how they move, how they formed and how they change over time.
Two closely related terms often come up. Astrophysics applies the laws of physics to explain how celestial objects work, such as why stars shine. Cosmology studies the universe as a whole: its origin in the Big Bang about 13.8 billion years ago, its structure and its future. In practice, most modern astronomers are also astrophysicists.
Astronomy is not astrology
People often mix up these two words. Astrology is the belief that the positions of the Sun, Moon and planets influence human personality and events. It has no scientific support: careful tests have not found that it predicts anything better than chance. Astronomy, by contrast, is a science that makes testable predictions, such as the exact timing of eclipses decades in advance. The two did share roots in ancient times, when sky-watching served calendars, navigation and religion alike, but they parted ways during the scientific revolution.
How astronomy began
People have watched the sky for as long as there have been people. Ancient monuments such as Stonehenge in England are aligned with sunrise and sunset at the solstices. Babylonian astronomers kept detailed records of planetary positions for centuries. Ancient Greek thinkers worked out that Earth is round and estimated its size. Astronomers in China recorded comets and “guest stars” (supernovae), and scholars in the medieval Islamic world refined instruments and star catalogues, which is why many bright stars, such as Aldebaran and Betelgeuse, have names derived from Arabic.
In the 1500s and 1600s, Nicolaus Copernicus proposed that Earth orbits the Sun, Johannes Kepler showed that planets move on ellipses, and Galileo Galilei turned a telescope to the sky, discovering Jupiter’s four largest moons and the phases of Venus. Isaac Newton then explained these motions with his law of universal gravitation. In the 20th century, Edwin Hubble showed that the universe contains many galaxies and is expanding.
How astronomers study things they cannot touch
Apart from a few samples of meteorites, Moon rocks and asteroid dust, astronomers cannot bring objects into a laboratory. Almost everything we know comes from studying light. Splitting light into a spectrum reveals what an object is made of, how hot it is and whether it is moving toward or away from us.
Visible light is only a small part of the story. Radio telescopes detect cold gas and pulsars; infrared telescopes such as NASA’s James Webb Space Telescope see through dust and look back to the first galaxies; X-ray telescopes reveal extremely hot gas around black holes. Space telescopes like Hubble and Webb work above the blurring and absorption of Earth’s atmosphere. In recent years astronomers have added entirely new messengers, including neutrinos and gravitational waves.
Who studies astronomy?
Professional astronomers work at universities, observatories and space agencies such as NASA and the European Space Agency (ESA). The International Astronomical Union (IAU), founded in 1919, brings together professional astronomers worldwide and is responsible for naming celestial objects and defining terms, such as the 2006 definition of a planet.
But astronomy is also one of the few sciences where amateurs make real contributions. Amateur astronomers discover comets and supernovae, track variable stars, time asteroid occultations and help confirm exoplanets. Online citizen-science projects let anyone classify galaxies, hunt for planets in telescope data or spot new asteroids from home.
Why astronomy matters
Astronomy answers some of the biggest questions people ask: where did we come from, and are we alone? The calcium in your bones and the iron in your blood were made inside stars that lived and died before the Sun formed. Astronomy has also produced practical spin-offs, from accurate timekeeping and navigation to imaging technology used in medicine and phone cameras, and spacecraft monitor space weather that can affect power grids and satellites.
How to get started
You do not need any equipment to begin. On the next clear night:
- Go outside, let your eyes adjust to the dark for 20 minutes, and find one constellation, such as Orion or the Big Dipper.
- Use a free sky chart or app to identify any bright planets that are up.
- Look at the Moon over a week and watch its phase change.
- Find a local astronomy club or public observatory night, where volunteers are often happy to share their telescopes.
From there, the rest of the Crash Course Astronomy series is a great free next step, as is NASA’s monthly skywatching guide.
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